Several years ago, I was advising a student on some code he had written for a data structures class. This code performed a set of actions on a temporary counter, and he was resetting the counter to zero after the last action so it would be ready for the next cycle of actions. I suggested setting the counter to zero just before the first action instead of just after the last action. He asked why, and this was my explanation.
This is also an example of what Jeanette Wing and others call computational thinking.
You know when you're doing laundry and you have to clean the lint out of the lint trap? Of course, this is not mandatory—the dryer will still work if you don't clean out the lint, but it'll work more efficiently if you do.
Now, you have two basic choices as to when to clean out the lint. You can clean it out when you remove clothes from the dryer, or you can clean it out when you put clothes into the dryer. Either protocol will work fine as long as everybody in the house is following the same protocol.
But suppose you have two people in the house following opposite protocols. Let's say that you follow the clean-after protocol and your housemate follows the clean-before protocol. If you do a load of laundry and clean out the lint trap when you're done, and then your housemate does a load of laundry, then the worst that will happen is that the lint trap will already be clean when your housemate goes to clean it.
On the other hand, if your housemate does a load of laundry and cleans out the lint trap at the beginning, and then you do a load of laundry, now you have a problem. When you go to clean out the lint when you're done, you'll find it extra full. You might even need to run the dryer a second time because your clothes didn't get completely dry.
The point is that some protocols, such as the clean-after protocol, only work if everybody follows the same protocol. Other protocols, such as the clean-before protocol, are more robust; they work fine—at least for you—even if some people follow the other protocol. If you can't guarantee that others will follow (or even remember!) a particular protocol, then you're better off choosing the robust protocol.
Here's a similar example. In college, I used to play a lot of bridge in the dorm lounge. In the hall right outside the lounge, there was a small bathroom. Now, there were two protocols concerning this bathroom. In the first protocol, you lock the door when using the bathroom; in the second protocol, you knock before entering. Somebody following the locked-door protocol may or may not knock, while somebody following the knocking protocol may or may not lock the door.
As long as everybody in the dorm follows the same protocol, there's no problem. But if different people follow different protocols, there can be tragic (or at least embarrassing) consequences!
In this example, perhaps the best course of action is to follow both protocols: lock the door and knock. The same is possible in many other situations where you want to be extra careful: clean the lint after drying but also check it again just before drying, initialize the counter just before the first action but also reset it to zero just after the last action, wear both a belt and suspenders.